The present invention relates to an ink jet head for ejecting ink to fly the ejected ink towards a recording medium, and an ink jet recording apparatus for recording an image corresponding to image data on a recording medium using the ink jet head.
An ink jet recording apparatus ejects ink from ejection ports to record an image corresponding to image data on a recoding medium. The ink jet recording apparatus, as well known, are classified into an electrostatic ink jet apparatus, a thermal ink jet apparatus, a piezoelectric ink jet apparatus and the like depending on a difference in ejection control means for ink.
Of those ink jet recording apparatus, the electrostatic ink jet recording apparatus uses ink containing charged colorant particles (colored charged particles). Thus, a predetermined voltage is applied to each of ejection portions of an ink jet head in correspondence to image data, thereby generating an electrostatic force in each of the ejection portions thereof, and an image corresponding to the image data is recorded on a recording medium by controlling the ejection of the ink by utilizing the electrostatic forces. As for such an electrostatic ink jet recording apparatus, an ink jet recording apparatus disclosed in JP 10-138493 A is known.
FIG. 7 illustrates a schematic constructional view of an example of the ink jet head of the ink jet recording apparatus disclosed in JP 10-138493 A. In an ink jet head 50 illustrated in FIG. 7, only one ejection portion of an ink jet head disclosed in JP 10-138493 A is conceptually illustrated. The ink jet head 50 includes a head substrate 51, an ink guide 52, an ejection port substrate (insulating substrate) 53, an ejection electrode (control electrode) 54, a counter electrode 55, a D.C. bias voltage source 58, and a pulse voltage source 59.
In the conventional electrostatic ink jet head as illustrated in JP 10-138493 A, the ejection electrode 54 is provided on a surface of the ejection port substrate 53. Hence, when a driving voltage is applied to the ejection electrode 54, an electric field Ed is generated not only from an upper surface of the ejection electrode 54, but also from a lower surface of the ejection electrode 54. That is, the electric field Ed directed from the ejection electrode 54 towards the surface of the head substrate 51 acts on ink Q circulating in an ink flow path 57. The electric field Ed generated in a direction from a lower surface of the ejection electrode 54 to the surface of the head-substrate 51 (hereinafter referred to as “repulsive electric field”) acts on colorant particles contained in the ink Q circulating in a main flow path so as to prevent the colorant particles contained therein from flowing into the ejection port (through hole) 56. Thus, when the ink jet head 50 is driven to apply a driving voltage to the ejection electrode 54, the colorant particles are prevented from being concentrated at the ejection port 56, and hence a given period of time is required until the colorant particles are sufficiently concentrated at the ejection port 56. For this reason, when an image is drawn at a high speed using such an ink jet head, there is encountered a problem in that the supply of the colorant particles to the ejection port 56 is impeded by the repulsive electric field generated from the ejection electrode 54 provided on the surface of the ejection port substrate 53, and hence dots each having a desired size cannot be stably formed on a recording medium.
In a case where in the electrostatic ink jet head, a plurality of ejection ports are arranged in matrix to construct a multi channel head, the distribution of wirings among the ejection electrodes of the ejection ports becomes gradually difficult. For this reason, when there are many channels, it is conceivable that the ejection port substrate is structured in the form of a multilayer wiring structure, and in this multilayer wiring structure, the wirings are connected to the ejection electrodes. Thus, when the number of channels is further increased in the future, it is necessary to further thicken the ejection port substrate. However, since if the ejection port substrate is thickened, a length of the ejection port increases as compared with an opening diameter of the ejection port, a resistance generated between the ink and an inner wall of the ejection hole increases so that the ink becomes difficult to be ejected. In addition, if the ejection port substrate is thickened, the ink flowing below the lower surface of the ejection port substrate may stay in the ejection port depending on the flow velocities of the ink. Thus, the ink becomes difficult to be supplied to a tip portion of the ink guide. As a result, a problem occurs in that the responsibility to an ejection frequency becomes poor, and hence as an image drawing frequency increases, the dot diameter becomes gradually small. Moreover, a problem also occurs in that when the ejection ports are integrated at high density along with an increase in the number of channels, a fluid interference with the adjacent channels is caused, and hence the diameters of the dots formed on the recording medium become unstable.
Note that when the ejection port substrate is thickened, that is, the length of the ejection port is increased, in the ink jet recording apparatus using the ink jet heads of the various types as well as in the electrostatic ink jet recording apparatus, the same problems as those of the foregoing arise.